The Misguided Silver Bullet: What XML will and will NOT do
to help Information Integration
Stuart E. Madnick1
ABSTRACT
The eXtensible Markup Language (XML) offers many important benefits and improvements over its
predecessor, HTML. But, articles have appeared about XML with exaggerated claims of it being a
"Rosetta Stone" with "miraculuous ways" to almost automatically provide information integration.
These claims are actually being believed by some executives. It is almost surprising that no one has
claimed that XML can cure cancer and provide world peace!
In reality, XML must face many of the same challenges that plagued Electronic Data Interchange
(EDI) and database integration efforts of the past. To a large extent, there are both managerial and
technical challenges – much related to the difficulties of attaining universally accepted
semantically-rich standards. In this paper, these challenges will be discussed with specific
emphasis on the issue of dealing with a real-world with multiple "contexts." Some promising
research directions, some overlapping with the "semantic web" effort, will be presented.
1. Introduction
The eXtensible Markup Language (XML) offers many important benefits and improvements over
its predecessor, HTML. Whereas once XML was merely described as “HTML on steroids,” articles
have appeared about XML with even more exaggerated claims of it being a “Rosetta Stone” 2 with
“a universal way to translate data”3 and “miraculous ways”4 to almost automatically provide
information integration. Some executives actually believe these claims. It is almost surprising that
no one has claimed that XML can cure cancer and provide world peace!
Before proceeding, it must be emphasized that XML does have real benefits and most of the
technical community, including the World Wide Web Consortium (W3C at www.w3.org), XML’s
originators, have taken a much more realistic perspective, recognize XML’s limitations (e.g., [10]),
and are working on further improvements [1]. The purpose of this article is to look at certain
aspects of information integration and understand XML’s capabilities and limitations.
In reality, XML must face many of the same challenges that plagued Electronic Data Interchange
(EDI) and database integration efforts of the past. To a large extent, there are both managerial and
technical challenges – much related to the difficulties of attaining universally accepted semantically
rich standards. In this paper, these challenges will be discussed with specific emphasis on the issue
of dealing with a real world with multiple "contexts." Some promising research directions, some
overlapping with the "semantic web" effort, will be presented.
1
Stuart E. Madnick, John Norris Maguire Professor of Information Technologies and Professor of Engineering Systems;
MIT Sloan School of Management; Cambridge, MA 02139; smadnick@mit.edu.
2
“Rosetta Stone,” Federal Computer Week, June 19, 2000; “A ‘Rosetta Stone’ for the Web?” Business Week Online,
June 14, 1999.
3
“XML: The Rosetta Stone for the Web,” Philly-Tech.com, April 2001 / Vol. 4.
4
“The Threat of XML,” ComputerWorld, July 9, 2001
2
2. Examples of Information Integration Applications and Requirements
“Information integration” is a term used to describe many different activities. For the purposes of
this paper, we will focus on a particular set of applications and requirements, often referred to as
“information aggregation.”
Two particularly popular current examples include “comparison” aggregators and “relationship” or
“account” aggregators. Aggregators with comparison capabilities are focused on collecting
information, especially prices, about specific products from multiple sources, primary online
merchants. Shopbots such as for those for purchasing books, music, and electronics are good
examples of this capability. These include MySimon (www.mysimon.com), C|net (www.cnet.com),
and DealTime (www.dealtime.com). Relationship aggregators focus on collecting information
related to the individual (or organization) rather than a product. Financial account aggregator
technology (e.g., www.yodlee.com) has been adopted by most major financial (e.g., Chase,
Citibank) and many non-financial institutions (e.g., CNBC, AOL). These organizations provide
their customers with the ability to manage all their financial relationships through a single
aggregator. For example, this includes the ability to see all of their account balances, from all
sources (e.g., bank accounts, brokerage accounts, credit cards, mortgages), integrated onto a single
web page. These comparison and relationship aggregators might operated intra-organizationally,
collecting information from multiple parts of a given enterprise (e.g., financial information from all
company divisions, manufacturing data from different plant locations) or might operate interorganizationally, combining information from multiple enterprises (e.g., price and account balance
information from multiple online sites.) A single aggregator may combine both relationship and
comparison capabilities for a given application.
It is important to note that in such applications, the primary and original purpose of the source sites
was not to support information aggregation. The individual online stores posted their product prices
for users visiting their site. The individual banks and other financial institutions made customer
account balances available online as a service and convenience for their customers. Although in
some cases direct data feeds and data exchange arrangements were made between source sites and
aggregators, in most cases the data was obtained from the sources using techniques often referred to
as “screen scraping” or “web wrapping.” These techniques involve the aggregator accessing the
source site as if it were a user (e.g., a browser) and then extracting the desired information from the
information provided (usually an HTML or XML page).
3. Benefits of XML
The benefits of XML have been described extensively in the literature (the list shown in Figure 1 is
adapted from [10]), so only a few key highlights will be discussed here. Probably one of the most
important benefits is that XML does help to create structured web pages, compared with HTML.
Feature
HTML
XML
Extensibility
Fixed set of tags
Extensible set of tags
Tag purpose
Tags describe presentation
Tags describe data content
Views
Single presentation
Multiple views of same document (by XSL)
Orientation
Documents
Documents plus semi-structured data
Search
Keyword search only
Keyword plus field-sensitive queries
Figure 1. Comparison of HTML and XML
3
In Figure 2(a) we see an example of an HTML page that might be returned when requesting price
information, in this case for a Palm Pilot V, from an online store. The HTML tags are used to
provide formatting information, such as margin sizes, font size, and such. The actual price
information might be simple text, as shown in Figure 2(a), or embellished with HTML tags defining
table delimiters and different font types, sizes, and/or colors for the different information (e.g.,
“Regular Price” in different color from “Our Price”). A considerable amount of programming effort
would be required to extract the price information from such a page in order to produce the desired
comparison aggregation of listing the corresponding prices for Palm Pilot V’s from multiple stores –
especially since it is likely that different formats will be used by different stores. Tools to support
and simply this effort, sometimes called “web wrappers,” have been developed [3].
(a) HTML
(b) XML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
<html>
<head> . . .
<BODY topmargin=18 leftmargin=6 bgcolor="#ffffff" link="#0000ee" VLINK="#551A8B"
ALINK="#ff0000">
<pre><font size=2>
<?xml version="1.0" ?>
<Document>
Palm Pilot V
Regular Our
Price
Price
329.00 236.00
In stock
</font></pre>
<table cellpadding=0 cellspacing=0 border=0>
<tr><td align=left valign=middle width=455 nowrap height=20>
<tr><td align=left valign=top nowrap width=455>
<font size=1 face="helvetica,arial"> . . .
</BODY>
</HTML>
<Product info>
<Product> Palm Pilot V <\Product>
<Regular price> 329.00 <\Regular price>
<Our price> 236.00 <\Our price>
<InStock> yes <\InStock>
<\Product info>
<\Document>
Figure 2. Product information using HTML and XML
In Figure 2(b), we see a possible XML page providing the same information. The actual formatting
of the page for presentation purposes is controlled by other facilities, such as eXtensible Stylesheet
Language (XLS) – and different online stores might use different stylesheets to produce very
different appearances of their data. For purposes of data extraction and aggregation of the price
information, the XML tags make the programming much easier. Furthermore, various XML parsers
are readily available to facilitate this process. Assuming all online stores used the same XML tags,
once the extraction procedure is established for one store, it should be fairly trivial to apply it to
many more stores. Thus, XML does, indeed, greatly facilitate such information integration.
4. XML Tends to Assume One Context
For purposes of information exchange and integration, to a large
extent, XML relies upon a generally agreed understanding of the
XML “tags”. That is, it assumes or hopes that these tags are all
understood in the same way by all of those sharing the
information. In cases where there is initial disagreement, it is
assumed that a “best way” will prevail or if there is not a way to
agree upon “best” than an acceptable compromise can be found.
This is a primary basis behind the concept of standards.
This standardization assumption can be a critical problem. For
example, consider Figure 3; is it a picture of an old lady or a
young lady? The point here is that some will see it one way, some
will see it the other way, and most be able to see both images –
but only one at a time. [If you are unable to see both, email me
and I will send clues for seeing each.] This is the situation that we
Figure 3. Old woman or young?
wwoman?
4
often face in real life. There is no “best” answer and different people will continue to see things in
different ways. Merely saying that everyone should see it the same way, does not change the reality
that multiple different legitimate, and often essential, views exist. Examples of this situation will be
presented in this paper.
5. XML Challenges
There are many challenges that XML must overcome in order to provide effective information
integration.
5.1 Multiple Standards
One of the key issues in using XML for information interchange or integration is the need for
consistent and standardized tags. There have been various efforts to create such standards in
various industries. Disputes over whether catalogs should use the tag “price” or “cost” must be
settled. Once agreed upon, these standardized tags can then become an effective way to exchange
information. Unfortunately, what is so great about XML standards – is that there are so many! For
example, the Director of Electronic Trading at Credit Suisse First Boston and Chairman of a
Financial Services XML Working Group was quoted as saying “there are more than a dozen XML
protocols – for Financial Trading applications alone.”5
To a certain extent, it is possible to map between the tags used in different XML standards using
eXtensible Stylesheet Language – Translations (XSLT) facilities which would help convert “price”
to “cost”, or vice versa. This can work well for fairly simple cases. But XSLT is primarily
designed for pair-wise translations, if there were n different sets of tags being used, there might
need to be n2 translations – which could be difficult if n were a large number and/or tag names
change periodically.
5.2 Rich Semantics
Let us assume that somehow all of these conflicting standards efforts converge and we all agree
upon a tag label “price” to be used in describing catalog information. But how precisely defined is
the notion of “price”? Is it in dollars ($) or pounds (£)? Even if it is “Dollars” is it US dollars,
Canadian dollars, Hong Kong dollars, or Singapore dollars? If we are dealing with international
commerce, these become important issues. There are various pragmatic ways to provide more
semantic information with the XML tags such as by having an auxiliary tag “currency” which
specifies what currency the price should be interpreted as. There are at least two problems with this
approach: (1) it is possible that the group developing the standard may have neglected to consider
the need for this “currency” modifier tag, specially if it was a group primarily dealing with domestic
commerce and (2) “currency” is only one of many semantic issues that must be addressed – will all
of them had been considered? For example, does the “price” include sales tax? (A key issue in the
United States) Does it include the value added tax (VAT) (an issue common in much of Europe)?
Does it include shipping charges? Etc. etc. etc.
One of the major challenges that Electronic Data Interchange (EDI) faced was coming up to a
sufficient level of detail to address all such cases. Because there can be so many nuances (and we
are not only considering the single issue of “price”) these efforts can go on for years – often without
reaching agreement. Even once agreements are sometimes reached, the end result is often of such
complexity that it becomes a major burden and difficulty to actually deploy. Once XML gets
5
“The Threat of XML,” ComputerWorld, July 9, 2001
5
beyond simple cases, it must also confront the same challenges. To illustrate how complex this can
become, consider figure 4, which represents actual data retrieved from the web.
Although the actual documents were
P/E Ratio
Dividend
encoded using HTML at the time, they all Source
used the heading “p/e ratio” to describe data ABC
11.6
0.29
displayed as shown. Presumably, a similar
5.57
8.127
XML tag would be used in the future. But, Bloomberg
you should notice that the four web sources, DBC
19.19
0.899
on the same day at approximately the same
7.46
0.47
time, had radically different values for “p/e MarketGuide
ratio”. How can these happen? Which one Figure 4. Key Financials for Daimler-Benz
is correct? The possibly surprising answer
is: they are all correct! The issue is, what do you really mean by “p/e ratio”. Some of these sites
even provide a glossary which gives a definition of such terms and they are very concise in saying
something like “p/e ratio” is “the price divided by the earnings”. As it turns out, this does not really
help us to explain the differences. The answer lies in the multiple interpretations and uses of the
term “p/e ratio” in financial circles. It is for the entire year for some but for others it is only for the
last quarter. Even when it is for a full year, is it the last four quarters? the last calendar year? the
last fiscal year? or the last three historical quarters and the estimated current quarter (a popular
usage)?
5.3 Evolution of Semantics
Even if one could agree upon a standard set of tags and even reach agreement upon the precise
meaning of each tag, there is also the problem of evolution. For example, in Europe most countries
are going through the conversion from using local currency (such as French Francs) to using Euros
as a common currency throughout Europe. Let us say that this transition takes place in France on
January 1, 2002. Thus, documents to be exchanged that originated prior to that date, will be
assuming local currency whereas documents created after that date will be assuming “Euros”.
Furthermore, not all organizations may make this change at the same time. This is a somewhat
simple case that could be resolved if there was a “currency” tag modifier being used – but in a more
general case, such as with “p/e ratio”, the nature of such a change may not be incorporated in the
preexisting tag set.
5.4 Multiple Purposes
Probably the fundamental challenge and difficulty with
attaining standards in general, is often there are multiple
purposes to be served for information exchange and these
different purposes necessitate different interpretations of
the information. For example, consider figure 5, which is
a list of organization names.
What is the relationship among these names? As it turns
out, these are all names that are in some way related to
each other and International Business Machines
Corporation (the name at the top of the list). These names
include abbreviations (such as IBM), divisions (such as
IBM Microelectronics division), wholly owned
subsidiaries (such as IBM Global Financing), partially
International Business Machines Corp.
IBM
IBM Microelectronics Division
IBM Global Services
IBM Global Financing
IBM Global Network
IBM de Colombia, S.A
Lotus Development Corporation
Software Artistry, Inc.
Dominion Semiconductor Company
MiCRUS
Computing-Tabulating-Recording Co.
Figure 5. List of organization names
6
owned subsidiaries (such as IBM de Colombia, S.A.), companies that were acquired by IBM (such
as Lotus Development Corporation), companies that were acquired and then later sold by IBM (such
as SoftwareArtistry, Inc.), companies in which IBM has a minority joint venture interest (such as
the Dominion Semiconductor company), and companies that IBM has a majority joint venture
interest in (MiCRUS). This list even includes IBM’s original name, Computing-TabulatingRecording Company!
What is the point of this discussion of these names? Well, let us consider a rather simple question:
“How many employees does IBM have?” In our recent study of integration activities in a major
insurance company, this was an example of an important question asked in setting premium rates
for business owner protection insurance [13]. In considering the entities listed in figure 5, which
ones should be included in this count? Also, how can one be sure that one is not also double
counting? In fact, the answer to the question itself depends upon the purpose of the question. The
important and subtle issue is,
Corporate household /family structure purposes when are two entities to be
- Financial: Risk ( credit - bankrupcy, country )
considered as part of the same
- Accounting: Account consolidation
entity. This issue is sometimes
- Marketing ( multiple divisions & subsidiaries ):
referred to as the Corporate
- Customers & Supplier consolidation ( economy of scale )
Household or Corporate Family
- Customer Relationship Management ( CRM)
Structure. As figure 6 illustrates,
- Managerial: Regional and/or Product separations
there are many different purposes
- Legal: Liability ( insurance )
for
which
such
corporate
- Relationship: Consultant Conflict of interest & competition
structures are used – in some cases
- Ad hoc/temp structures
two entities should be combined
- and these are dynamic, changing over time
and in other cases the two entities
Figure 6. Examples of multiple purposes
should not be combined.
In fact, figure 6 does not illustrate all the subtlety and complexity that may be involved. For
example, a few years ago when Russia announced that it would suspend payments on its financial
obligations, all of the major financial organizations were immediately concerned to know how much
money they had at risk. (Although not a daily occurrence, similar problems do occur quite
regularly, most recently involving financial instability in Argentina, previously Brazil, Asia a few
years ago, and the threaten financial instability of Long Term Capital Management Corp, are other
examples). In considering the case of the Russian risk, there are many complexities. The obvious
cases are where the financial institution had directly loaned money to a Russian company or
government organization. But, in fact, there are subtleties here. If, for example, the Russian
company is a wholly owned subsidiary of a US Company, then the US company is obligated to be
responsible for the debts of its subsidiary. Thus, this particular Russian company would not be
included in the risk assessment, especially if the consideration is regarding bankruptcy. Likewise
there can be considerations of liability in case of an industrial accident; there are complex laws that
affect which entities will be held responsible.
Also as noted before, the corporate structures change over time, thereby also changing the context
over time. Thus, at one point Lotus Development Corporation was a separate corporation. When
doing a historical comparison of growth or decline in “number of employees” should the current
Lotus employees be counted in a total as of today? Should the Lotus employees in 1980, when it
was a separate corporation, be added with the IBM employees of 1980 to make a meaningful
comparison?
7
6. Role of Context
In our work at MIT we have considered the importance of Context in interpreting information.
Most information, no matter how carefully defined, usually has incomplete specifications. Some
information is “assumed” by the users of that information. This is not a problem if the information
and the people who use it are closely coupled together and share a common “context”. For
example, if an MIT student asked me what course I was teaching this year and I say “15.561” – a
meaningful exchange of information has taken place. To an outsider who is not familiar with the
MIT numbering conventions and meaning of the various courses, this information would not be
particularly meaningful. The challenge we face is that the Internet and World Wide Web have made
it possible to electronically gather documents from around the world, but the context is often left
behind.
To illustrate the significance of this issue, consider the vignettes displayed in Figure 7(a) and Figure
7(b). In the case of Figure 7(a), the emissaries of the Austrian and Russian emperors thought that
they had agreed on the battle being “October 20th”. What they had not agreed upon was which
October 20th! To illustrate that this kind of semantic misunderstandings do not only resided hundred
of years in the past, consider Figure 7(b) where a similar mishap also had dramatic consequences for
the Mars Orbiter satellite.
(a) The 1805 Overture
In 1805, the Austrian and Russian Emperors agreed to join forces against Napoleon. The
Russians promised that their forces would be in the field in Bavaria by Oct. 20.
The Austrian staff planned its campaign based on that date in the Gregorian calendar.
Russia, however, still used the ancient Julian calendar, which lagged 10 days behind.
The calendar difference allowed Napoleon to surround Austrian General Mack's army at
Ulm and force its surrender on Oct. 21, well before the Russian forces could reach him, ultimately
setting the stage for Austerlitz.
Source: David Chandler, The Campaigns of Napoleon, New York: MacMillan 1966, pg. 390.
(b) The 1999 Overture
Unit-of-Measure mixup tied to loss of $125 Million Mars Orbiter
“NASA’s Mars Climate Orbiter was lost because engineers did not make a simple
conversion from English units to metric, an embarrassing lapse that sent the $125 million craft off
course … The navigators [JPL] assumed metric units of force per second, or newtons. In fact, the
numbers were in pounds of force per second as supplied by Lockheed Martin [the contractor].”
Source: Kathy Sawyer, Boston Globe, October 1, 1999, pg. 1.
Figure 7. Examples of consequences of misunderstood context
7. Context Interchange Architecture
The shortcomings of XML alone for information integration and meaningful interpretation have
been recognized by others. The work by the W3C on the Resource Description Framework (RDF)
and the “Semantic Web” [1] are intended to address some of these issues.
The COntext INterchange (COIN) project at MIT is also addressing these needs through a
mediation approach for semantic integration of disparate (heterogeneous and distributed)
information sources [5,6,7,11,12]. The overall COIN project includes not only the mediation
infrastructure and services, but also wrapping technology and middleware services for accessing the
source information and facilitating the integration of the mediated results into end-users
applications. The wrappers are physical and logical gateways providing a uniform access to the
disparate sources over the network [3].
8
The set of Context Mediation Services comprises a Context Mediator, a Query Optimizer, and a
Query Executioner. The Context Mediator is in charge of the identification and resolution of
potential semantic conflicts induced by a query. This automatic detection and reconciliation of
conflicts present in different information sources is made possible by general knowledge of the
underlying application domain, as well as informational content and implicit assumptions associated
to the receivers and sources. These bodies of declarative knowledge are represented in the form of a
domain model, a set of elevation axioms, and a set of context theories respectively.
The result of the mediation is a mediated query. To retrieve the data from the disparate information
sources, the mediated query is then transformed into a query execution plan, which is optimized,
taking into account the topology of the network of sources and their capabilities. The plan is then
executed to retrieve the data from the various sources; results are composed as a message, and sent
to the receiver.
The Context Interchange (COIN) approach allows queries to the sources to be mediated, i.e.,
semantic conflicts to be identified and solved by a context mediator through comparison of contexts
associated with the sources and receivers concerned by the queries. It only requires the minimum
adoption of a common Domain Model, which defines the domain of discourse of the application.
CONTEXT MEDIATION SERVICES
Conversion
Library
Local Store
Length
Meters /Feet
Domain
Model
Domain
USERS and
APPLICATIONS
Context
Axioms
Feet
Query Plan
Mediated
Query
Executioner
Optimizer
Query
Context
Mediator
Subqueries
Wrapper
Elevation
Axioms
Context
Axioms
DBMS
Wrapper
Elevation
Axioms
Context
Axioms
Meters
Semi-structured
Data Sources
(e.g., XML)
Figure 8. The Architecture of the Context Interchange System
The knowledge needed for integration is formally modeled in a COIN framework [4] as depicted in
Figure 8. The COIN framework is a mathematical structure offering a sound foundation for the
realization of the Context Interchange strategy. The COIN framework comprises a data model and a
language, called COINL, of the Frame-Logic (F-Logic) family [2,9]. The framework is used to
define the different elements needed to implement the strategy in a given application:

The Domain Model is a collection of rich types (semantic types) defining the domain of
discourse for the integration strategy (e.g., “Length”);

Elevation Axioms for each source identify the semantic objects (instances of semantic types)
corresponding to source data elements and define integrity constraints specifying general
properties of the sources;
9

Context Definitions define the different interpretations of the semantic objects in the different
sources or from a receiver's point of view (e.g., “Length” might be expressed in “Feet” or
“Meters”).
Finally, there is a conversion library which provides conversion functions for each modifier to
define the resolution of potential conflicts. The conversion functions can be defined in COINL or
can use external services or external procedures. The relevant conversion functions are gathered and
composed during mediation to resolve the conflicts. No global or exhaustive pair-wise definition of
the conflict resolution procedures is needed.
Both the query to be mediated and the COINL program are combined into a definite logic program
(a set of Horn clauses) where the translation of the query is a goal. The mediation is performed by
an abductive procedure which infers from the query and the COINL programs a reformulation of the
initial query in the terms of the component sources. The abductive procedure makes use of the
integrity constraints in a constraint propagation phase which has the effect of a semantic query
optimization. For instance, logically inconsistent rewritten queries are rejected, rewritten queries
containing redundant information are simplified, and rewritten queries are augmented with auxiliary
information. The procedure itself is inspired by the Abductive Logic Programming framework [8]
and can be qualified as an abduction procedure. One of the main advantages of the abductive logic
programming framework is the simplicity in which it can be used to formally combine and to
implement features of query processing, semantic query optimization and constraint programming.
8. Conclusion
We are in the midst of exciting times – the opportunities to access and integrate diverse information
sources over the web are incredible but the challenges are considerable. XML can greatly facilitate
this process by providing more syntactic structure to web documents. But the effective use of
semantic metadata and context knowledge processing is needed to enable us to overcome the
challenges described in this paper and more fully realize the opportunities. A particularly
interesting aspect of the context mediation approach described is the use of context metadata to
describe the expectations of the receiver as well as the semantics assumed by the sources.
ACKNOWLEDGEMENTS
Work reported herein has been supported, in part, by the USA Advanced Research Projects Agency,
Banco Santander Central Hispano, Citibank, Fleet Bank, FirstLogic, Merrill Lynch, MIT Total Data
Quality Management (TDQM) Program, PricewaterCoopers, Suruga Bank, and USAF/Rome
Laboratory.
Information about the Context Interchange project can be obtained at
http://context2.mit.edu.
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